Contents

History

The name most often associated with emission theory is Isaac Newton. In his
Corpuscular theory Newton visualised
light "corpuscles" being thrown off from hot bodies at a nominal
speed of c with respect to the emitting object, and obeying the
usual laws of Newtonian mechanics (although he did also assign wave
properties to light).

Special relativity's geometrical simplicity was persuasive, but
a convincing general disproof of emission theory proposed was still
difficult to find, and some considered the main competitor to
Einstein's special theory to be the emission theory proposed by Walter
Ritz.

Many years later R.S. Shankland reports Einstein as saying that
Ritz' theory had been "very bad" in places and that he himself had
eventually discarded emission theory because he could think of no
form of differential equations that described it, since it leads to
the waves of light becoming "all mixed up".

In 1913 Willem de Sitter wrote that the
expected consequences of emission theory on the appearance of
double stars, an extreme scrambling of their lightsignals, did not
happen. This was widely accepted as definitive proof that emission
theory was not viable.

Problems with emission
theory

The simplest form of emission theory says that radiating objects
throw off light with a speed of "c" relative to their own state of
motion, and (unless we have reason to believe that the light
changes speed in flight), we then expect light to be moving towards
us with a speed that is offset by the speed of the distant emitter
(c ± v) ). This description generates three "odd"
results:

If a radiant star moves across our field of vision, light given
off by differently-moving atoms in its atmosphere should take
different amounts of time to reach us. Since the retreating atoms
would have a "red" Doppler shift, and the approaching ones a
"blue" Doppler shift, the passing star might be expected to appear
as a "rainbow streak".

Similarly, if a radiant star is eclipsed, one might expect the
eclipsing shadow to appear to intercept different colours of
Doppler-shifted light in sequence - the eclipse might appear to
have coloured fringes.

For the case of a double-star system seen edge-on, light from
the approaching star might be expected to travel faster than light
from its receding companion, and overtake it. If the distance was
great enough for an approaching star's "fast" signal to catch up
with and overtake the "slow" light that it had emitted earlier when
it was receding, then the image of the star system should appear
completely scrambled.

De Sitter argued that
none of the star systems he had studied showed the extreme optical
effect behaviour in [3], and this was considered the death knell
for Ritzian theory and emission theory in general.

Newton appears to have enquired whether or not moons of Jupiter
showed coloured fringes at eclipse, suggesting that he may have
already been aware of these arguments and problems.

Variations
on a theme

The idea that perhaps the speed of light only has an effective
value of cEMITTER while it is local to the
emitter, as a "light-dragging" or "proximity" effect has been
considered in detail. This can be expressed in terms of the
"extinction effect", and it arguably undermines the cogency of
deSitter type evidence based on optical stars. However, similar
observations have been made more recently in the x-ray spectrum,
which have a long enough extinction distance that it should not
affect the results. The observations confirm that the speed of
light is independent of the speed of the source. In addition,
terrestrial experiments have been performed, over very short
distances, where no "light dragging" or extinction effects could
come into play, and again the results confirm that light speed is
independent of the speed of the source, conclusively ruling out
emission theories.

Furthermore, quantum electrodynamics places the propagation of
light in an entirely different, but still relativistic, context,
which is completely incompatible with any theory that postulates a
speed of light that is affected by the speed of the source.